1. Field of the Invention
The invention relates to a circuit for providing power to a voltage regulator at start-up.
2. Description of the Related Art
Voltage regulator circuits typically convert an unregulated DC input voltage into a regulated DC output voltage of a predetermined value. Voltage regulators in frequent use include pulse width modulation (PWM) switching converters such as those described in U.S. Pat. Nos. 4,236,198; 4,686,616; 4,695,936; and 4,890,211, incorporated herein by reference, pulse frequency modulation (PFM) switching converters such as those described in U.S. Pat. Nos. 5,568,044 and 5,801,518, incorporated herein by reference, resonant switching converters such as those described in U.S. Pat. Nos. 5,546,294 and 5,694,304, incorporated herein by reference, and linear converters such as that described in U.S. Pat. No. 4,644,247, incorporated herein by reference.
Typically, voltage regulation is accomplished by a voltage regulator control circuit that monitors the output voltage and uses feedback to keep the output voltage constant. Generally, the voltage regulator control circuit itself must be powered by a regulated voltage. When the voltage regulator is operating, the control circuit may advantageously be powered by a regulated voltage derived from the output of the voltage regulator. However, there is a transient period when the voltage regulator is turned on during which the voltage regulator is not providing a steady regulated output voltage and cannot power the voltage regulator control circuit.
Start-up circuits are known that switch the voltage regulator operating voltage from the power supply voltage to the regulated voltage. However, these start-up circuits suffer from various drawbacks depending on the type of circuit. Some drawbacks include their use of high value resistors and power consumption even after the start-up period.
What is needed is a start-up circuit for voltage regulators that does not suffer from the drawbacks of prior art start-up circuits.